Working from foggy and often faulty memory, refreshed by a tour of B-Rad's site:
Obviously it can't be called simply "app.bin" because you may want more than one. I think any valid filename that ends in ".app.bin" is OK. "pure-ftpd.app.bin" is definitely OK since that's the name B-Rad used in his examples.
Put the file in the root directory of a USB drive, usually a flash drive, formatted as FAT32, Ext2 or Ext3 but not NTFS. Then reboot. A hard drive might work if it spins up fast enough during boot. If the drive is also your media storage drive, note that FAT32 has a 4GB file size limit.
Default ID is "root" with no password. Some FTP clients won't work with a blank password.
Many more tips on the
WDLXTV App pack: pure-ftpd.app.bin-0.1 page.